CMOS image sensors are emerging as a viable alternative to CCD sensors due to the low power consumption and high integration capability of CMOS circuitry. However, CMOS imaging sensors also have various problems. One example is the so-called fixed-pattern-noise (FPN) caused by device mismatches and/or process nonuniformities. A mismatch occurs at each pixel site, and at each column read-out.
An example of a known CMOS imaging sensor is shown in FIG. 9. The key blocks are: Pixel Block; Column Block; and Chip Output Block. The pixel Block (one for each pixel) includes the following: Photodiode PD; NMOS Transistor N1; and Switches RES and SEL. The Column Block (one for each column of Pixels) includes the following: Capacitors C1 and C2; PMOS Transistor P1; Switches CDS and COL; and Current sources IPIXEL and ICOL. The Chip Output Block (one for the whole chip) includes the following: PMOS Transistor P2; Switch CHIP; and Current Source ICHIP.
The operation of the Pixel Block is as follows: Node IN is connected to switch RES, the cathode of photodiode PD, and the gate of NMOS transistor N1. Initially switch RES is closed and the voltage on node IN is VRES. Then switch RES is opened. There will be a finite charge on node IN dependent on the voltage VRES, the capacitance of photodiode PD, and the gate capacitance of NMOS transistor N1. The photodiode current causes the charge on node IN to be discharged and the voltage on node IN decreases. Generally imagers have a fixed integration time or period. The voltage on node IN at the end of the integration period is referred to herein as VPD.
The voltage on node IN is read out using NMOS transistor N1 and Switch SEL, the Column Block circuit, and the Chip Output Block circuit.
FIG. 10 summarizes the position of the switches during the Integration Period and the Pixel Readout, which enables the FPN to be suppressed.
During the Integration Period, RES and SEL are open. During the Pixel Readout, the following occurs.
Readout Step 1: RES and SEL are open, CDS, COL, and CHIP are closed to reset the Column and Chip Blocks. The voltage across C1 will be zero. The voltage across C2 is VP1gs, which is the gate to source voltage of PMOS transistor P1.
Readout Step 2: SEL is closed and COL is opened. The voltage across C1 becomes VPD−VN1gs (VN1gs=gate to source voltage of NMOS transistor N1). The voltage across C2 remains VP1gs.
Readout Step 3: CDS and CHIP are opened. The voltage across C1 remains VPD−VN1gs. The voltage across C2 remains VP1gs.
Readout Step 4: RES and COL are closed. The source voltage of N1 becomes VRES−VN1gs. The voltage across C1 remains VPD−VN1gs. Thus the gate voltage of P1 becomes (VRES−VN1gs)−(VPD−VN1gs)=VRES−VPD. The source voltage of P1 becomes (VRES−VPD)−VP1gs. The voltage across C2 remains VP1gs. Thus the gate voltage of P2 becomes (VRES−VPD)−VP1gs+VP1gs=VRES−VPD. The readout voltage OUT is VRES−VPD+VP2gs where VP2gs is the gate to source voltage of PMOS transistor P2. PMOS transistor P2 is a common device used for the readout of all pixels.
Both VN1gs and VP1gs terms are canceled in this Sequential Correlated Double Sampling Technique. The N1 and P1 Vt terms, which are embedded in the VN1gs and VP1gs, are also canceled. Thus the effect of CMOS Vt mis-matches are suppressed with the above technique and the Fixed Pattern Noise is greatly reduced.
Readout Step 5: CHIP is closed. The readout voltage OUT equals VP2gs. The rest of the switches are opened. The pixel has been reset for the next Integration Period. The system is ready for the next pixel readout.
The above description is a readout operation for one pixel. During the Integration Period for one pixel, the Column Block and Chip Output Blocks are being used for Readout of other pixels.
Some problems with the CMOS imaging sensor of FIG. 9 include the disadvantageous effect of parasitic routing capacitance caused by capacitors C2 (thousands of them in a complete pixel array) driving the transistor P2, and the fact that the capacitors are typically poly/n-well capacitors which disadvantageously tend to be stray-sensitive and also suffer from a leakage problem.
It is desirable in view of the foregoing to provide for CMOS image sensing that avoids the aforementioned problems associated with known CMOS imaging sensors.
According to the invention, a single capacitor can be used for both readout and reduction of device mismatches. Such dual-purpose use of a single capacitor is facilitated by a switching arrangement. The switching arrangement connects the capacitor to a low impedance node during charge storage, thereby advantageously providing the stored charge with a stray-insensitive, leakage independent characteristic. Also, the column readout line is driven by the low impedance node, thereby advantageously reducing parasitic routing capacitance.